The present invention relates to a method and/or architecture for demodulating digital video broadcast (DVB) signals.
There are currently two major types of DVB, namely, terrestrial broadcasting and satellite/cable broadcasting. The invention is particularly, though not exclusively concerned with terrestrial broadcasting, which has special problems, particularly in communication channel impairment, arising from adjacent television channels, multipath, and co-channel interference, for example. A type of transmission which has been developed to meet these problems is known as Coded Orthogonal Frequency Division Multiplexing (COFDM)xe2x80x94see for example xe2x80x9cExplaining Some of the Magic of COFDMxe2x80x9d Stott, J. H.xe2x80x94Proceedings of 20th International Television Symposium, Montreux, June 1997. In COFDM, transmitted data is transmitted over a large number of carrier frequencies (1705 or 6817 for DVB) occupying a bandwidth of several MHz in the UHF spectrum, spaced (by the inverse of the active symbol period) so as to be orthogonal with each other; the data is modulated as QPSK or QAM and convolutionally coded, to enable soft-decision (Viterbi) decoding. Metrics for COFDM include Channel State Information (CSI) which represents the degree of confidence in each carrier for reliably transmitting data.
Modulation and Demodulation of the carriers may be carried out by a Fast Fourier Transform (FFT) algorithm performing Discrete Fourier Transform operations. Naturally, various practical problems arise in demodulation, firstly in translating the transmitted signal to a frequency at which demodulation can be carried out, and secondly by accurately demodulating the data from a large number of carriers in a demodulator which is not overly complex or expensive, which involves inter alia synchronizing the demodulator (receiver) in time to the incoming signal. This is important for the proper execution of the FFT algorithms.
The data signal on each carrier has a relatively long symbol period and this, in part, gives the signal its good performance in conditions of multipath propagation. The multipath performance is further enhanced by the inclusion of a guard interval in which a portion of the modulated signal waveform taken from the end of each symbol is also included at the beginning of the symbol period. Different fractions of the basic symbol period can be used in this way to provide immunity to multipath distortion of increasingly long delays.
The principal requirement for synchronization in a receiver is to obtain from the signal waveform a reliable time synchronization pulse related to the start of the symbol period. Such a pulse is then used to start, at the correct position in the waveform, the process of Fourier Transformation which accomplishes a major portion of the demodulation process. A second requirement for synchronization is to lock a digital sampling clock for analog to digital conversion in the receiver to an appropriately chosen harmonic of the symbol period. However, the modulated OFDM waveform produced by adding together all the modulated carriers is essentially noise-like in nature and contains no obvious features such as regular pulses which could be used to synchronize the circuitry receiver. Because of this, techniques for synchronization are based on correlation of the signal with a version of itself delayed by the basic symbol period. The similarity between the portion included to form the guard interval and the final part of the basic symbol is then shown as a region of net correlation while the remainder of the symbol period shows no correlation. Even so, the correlated waveform still reflects the noise-like nature of the signal waveform and can be impaired by signal distortions, so it is necessary to process the signal further to obtain reliable synchronization.
British patent application GB-A-2037155 describes time synchronization involving the use of correlation and a filter which exploits the periodicity of the waveform to form a complex symbol pulse. The modulus of the-pulse signal is used to derive a pulse related to the start of the symbol period and a signal to control a sampling clock frequency in the demodulator.
An implementation of such an arrangement in the 8K mode of COFDM would require about 369 Kbits of memory, which is far too great for implementation in a single chip.
It is an object of the present invention to provide a demodulator for digital terrestrial broadcast signals which can demodulate data transmitted by a COFDM system but which my be manufactured simply and inexpensively, preferably in a single integrated circuit chip.
The present invention is based on the recognition that in addition to timing synchronization, correct demodulation requires inter alia both automatic frequency control (AFC) and channel equalization (CE), both of which have memory requirements, but neither of which processes can be initiated until the timing synchronization process is locked. The present invention envisages using the memory intended for use by AFC and/or CE in an initial hunt mode to establish synchronization where a large amount of memory is required to hunt over a wide timing range, and then subsequently maintaining synchronization in a zoom mode, where only a narrow timing range about the synchronization point is checked for timing variation, requiring a small amount of memory. In the zoom mode, AFC and CE may come into operation, making use of the memory no longer required by timing. Should synchronization be lost in the zoom mode, the system reverts to hunt mode, with suspension of AFC and CE.
Usually, the relatively wide timing range will be a symbol period, preferably a full OFDM sampling period.